In order to meet increasingly stringent federal regulations of NOx and other undesirable emissions, engineers are constantly seeking new strategies of reducing the undesirable emissions. One method of reducing NOx emissions is NOx selective catalytic reduction (SCR) systems. These systems use ammonia (NH3) to reduce NOx to nitrogen (N2) and water. Although these systems can reduce NOx emissions, NOx selective catalytic reduction systems often require ammonia storage on the vehicle. Ammonia tanks can consume valuable space within the engine system and must be replenished periodically. Further, because of the high reactivity of ammonia, on-board storage of the ammonia can be hazardous.
Some of the drawbacks associated with the use of NOx selective catalysts can be eliminated by the use of on-board ammonia generation systems. For instance, the on-board ammonia production system set forth in U.S. Pat. No. 6,047,542, issued to Kinugasa on Apr. 11, 2000, injects an increased amount of fuel into one cylinder group within a plurality of cylinders in order to create a rich exhaust from the one cylinder group. The rich exhaust is then passed over an ammonia-producing catalyst that converts a portion of the NOx in the rich exhaust into ammonia. It has been found that the efficiency of conversion of NOx to ammonia by the ammonia-producing catalyst may be improved under rich conditions. The exhaust and the ammonia is then combined with the exhaust from a second cylinder group and passed through a SCR catalyst where the ammonia reacts with NOx to produce nitrogen gas and water.
Although the Kinugasa method allows for on-board generation of ammonia, the different operations of the cylinders can create drawbacks. For instance, an engine may function less efficiently and with lower power output when rich combustion occurs in a portion of the cylinders. Moreover, the two cylinder groups, operating in the Kinugasa method, may cause significant power imbalance within the engine, resulting in engine vibrations.
The present disclosure is directed at overcoming one or more of the problems set forth above.